GUID Partition Table

Diagram illustrating the layout of the GPT scheme. In this example, each logical block is 512 bytes in size, and each partition entry is 128 bytes, and the corresponding partition entries are assumed to be located in LBA 2-33, here. LBA addresses that are negative indicate position from the end of the volume, with −1 as the last addressable block.

The widespread MBR partitioning scheme, dating from the early 1980s, imposed limitations that affect the use of modern hardware. One of the main limitations is the usage of 32 bits for storing block addresses and quantity information. For hard disks with 512-byte sectors, the MBR partition table entries allow up to a maximum of 2 TiB (232×512 Bytes).[1]

Intel therefore developed a new partition table format in the late 1990s as part of what eventually became UEFI. As of 2010[update], GPT forms a subset of the UEFI specification.[2] GPT allocates 64 bits for logical block addresses, therefore allowing a maximum disk size of 264 sectors. For disks with 512-byte sectors, maximum size is 9.4 ZB (9.4 × 1021 bytes) or 8 ZiB (9,444,732,965,739,290,427,392 bytes, coming from 18,446,744,073,709,551,616 (264) sectors × 512 (29) bytes per sector).[1][3]

MBR-based partition table schemes insert the partitioning information for (usually) four "primary" partitions in the master boot record (MBR) (which on a BIOS system is also the container for code that begins the process of booting the system). In a GPT, the first sector of the disk is reserved for a "protective MBR" such that booting a BIOS-based computer from a GPT disk is supported, but the boot loader and operating system must both be GPT-aware. Regardless of the sector size, the GPT header begins on the second logical block of the device.

Like modern MBRs, GPTs use logical block addressing (LBA) in place of the historical cylinder-head-sector (CHS) addressing. The protective MBR is contained in LBA 0, the GPT header is in LBA 1, and the GPT header has a pointer to the partition table, or Partition Entry Array, typically LBA 2. The UEFI specification[4] stipulates that a minimum of 16,384 bytes, regardless of sector size, be allocated for the Partition Entry Array. On a disk having 512-byte sectors, a partition entry array size of 16,384 bytes and the minimum size of 128 bytes for each partition entry, LBA 34 is the first usable sector on the disk.

Hard disk manufacturers are transitioning to 4,096-byte sectors. As of 2010, the first such drives continue to present 512-byte physical sectors to the OS, so degraded performance can result when the drive's (hidden) internal 4 KiB sector boundaries do not coincide with the 4 KiB logical blocks, clusters and virtual memory pages common in many operating systems and file systems. This is a particular problem on writes when the drive is forced to perform two read-modify-write operations to satisfy a single misaligned 4 KiB write operation.[5] Such a misalignment occurs by default if the first partition is placed immediately after the GPT, as the next block is LBA 34, whereas the next 4 KiB boundary begins with LBA 40.

For backward compatibility with most legacy operating systems like DOS, OS/2 and versions of Windows before Vista, MBR partitions must always start on track boundaries according to the traditional CHS addressing scheme and end on a cylinder boundary. This even holds true for partitions with emulated CHS geometries (as reflected by the BIOS and the CHS sectors entries in the MBR partition table) or partitions accessed only via LBA. Extended partitions always start on cylinder boundaries as well.

This typically causes the first primary partition to start at LBA 63 on disks accessed via LBA, leaving a gap of 62 sectors with MBR-based disks, sometimes called "MBR gap", "boot track", or "embedding area". That otherwise unused disk space is commonly used by boot loaders such as GRUB for storing their second stages.[6] On older computers using alternative LBA/CHS translation schemes or different extended CHS mappings, with smaller LBA-accessed disks, or on disks accessed via CHS only, the gap could be smaller, although not normally less than LBA 16 on normal hard disks.

Since Windows Vista, the first partition usually starts after a gap of 2,047 sectors at LBA 2,048 as part of its new 1 MiB partition alignment policy, so no large-sector misalignment occurs by default, but serious compatibility problems with older operating systems and disk tools exist.

GPT also provides redundancy, writing the GPT header and partition table both at the beginning and at the end of the disk.

If the minimum size of 16,384 bytes is allocated for the partition entry array, and the default size of 128 bytes is used for each partition entry, then the maximum number of partitions is limited to 128.

Traditionally, in IBM PC compatible systems the first sector of the disk holds the Master Boot Record (MBR), containing the drive's partitioning information and the code of the first stage boot loader for BIOS-based systems. For limited backward compatibility, this sector is still reserved for an MBR in the GPT specification, but it is now used in a way that prevents MBR-based disk utilities from misrecognizing and possibly overwriting GPT disks. This is referred to as a protective MBR.[3]

A single partition type of EEh, encompassing the entire GPT drive (where "entire" actually means as much of the drive as can be represented in an MBR), is indicated and identifies it as GPT. Operating systems and tools which cannot read GPT disks will generally recognize the disk as containing one partition of unknown type and no empty space, and will typically refuse to modify the disk unless the user explicitly requests and confirms the deletion of this partition. This minimizes accidental erasures.[3] Furthermore, GPT-aware OSes may check the protective MBR and if the enclosed partition type is not of type EEh or if there are multiple partitions defined on the target device, the OS may refuse to manipulate the partition table.[7]

While the MBR layout (and also the protective MBR layout) was defined around a sector size of 512 bytes per sector, the actual sector size can be larger on various media such as MO disks or hard disks with Advanced Format. Extra space in the MBR typically remains unused.

If the actual size of the disk exceeds the maximum partition size representable using the legacy 32-bit LBA entries in the MBR partition table, the recorded size of this partition is clipped at the maximum, thereby ignoring the rest of disk. This amounts to a maximum reported size of 2 TiB, assuming a disk with 512 bytes per sector (see 512e). It would result in 16 TiB with 4 KiB sectors (4Kn), but since many older operating systems and tools are hard-wired for a sector size of 512 bytes or are limited to 32-bit calculations, exceeding the 2 TiB limit would cause serious compatibility problems.[3]

In operating systems that support GPT-based boot through BIOS services rather than EFI, the first sector is also still used to store the first stage of the bootloader code, but modified to recognize GPT partitions. The boot loader in the MBR must not assume a fixed sector size of 512 bytes / sector.[3]

Apple's Boot Camp software for Intel-based Apple Macs creates a hybrid partition table to allow the booting of Windows (which at the time of Boot Camp's creation did not support GPT or EFI). In this system the protective partition is reduced in size to cover from sector 1 to the sector before the first regular partition included in the hybrid MBR. Additional MBR partitions are then defined to correspond to the next three GPT[citation needed] partitions.

The partition table header defines the usable blocks on the disk. It also defines the number and size of the partition entries that make up the partition table. The EFI stipulates a minimum of 16,384 bytes be reserved for the partition table array, so there are 128 partition entries reserved, each 128 bytes long.

The header contains the disk globally unique identifier (GUID). It records its own size and location (always LBA 1!) and the size and location of the secondary GPT header and table (always the last sectors on the disk). Importantly, it also contains a CRC32 checksum for itself and for the partition table, which may be verified by the firmware, bootloader and/or operating system on boot. Because of this, hex editors should not be used to modify the contents of the GPT. Such modification would render the checksum invalid. In this case, the primary GPT may be overwritten with the secondary one by disk recovery software. If both GPTs contain invalid checksums, many bootloaders (those governed by an integrity model in particular) and operating systems will refuse to work with the disk until the corrupted partition tables are repaired or removed.

Reserved; must be zeroes for the rest of the block (420 bytes for a sector size of 512 bytes; but can be more with larger sector sizes)

LBA size

Total

The values for current and backup LBAs of the primary header should be the second sector of the disk (LBA 1) and the last sector of the disk, respectively. The secondary header at the end of the disk identifies its own table of partition entries, which is located directly before that header.

This table must be referenced relative to LBA 1. Thus, on 4Kn disks with 4 KiB sectors it does not follow the 512 bytes of the MBR physically (stored in LBA 0) and thereby becomes part of LBA 0 on disks with larger sector sizes. While the described arrangement happens to occur on disks with 512 bytes per sector, there may be "gaps" of unused space between them on disks with larger sector sizes. In other words, with 512-byte sectors it starts at 512th byte from the beginning of the disk, while with 4 KiB sectors it starts at 4096th byte. If multi-sector reads are performed, the actual sector size must be included in the calculation when referencing this table.

The GPT uses simple and straightforward entries to describe partitions. The first 16 bytes designate the partition type globally unique identifier (GUID). For example, the GUID for an EFI System partition is C12A7328-F81F-11D2-BA4B-00A0C93EC93B. The second 16 bytes contain a GUID unique to the partition. Then follow the starting and ending 64-bit LBAs, partition attributes and partition names. As is the nature and purpose of GUIDs, no central registry is needed to ensure the uniqueness of the GUID partition type designators. The location of the partition entries array on disk is defined in the GPT header.

The GPT header contains a field that specifies the size of a partition table entry. The minimum required is 128 bytes, but implementations must allow for other values.[8]

Also, the sector size must not be assumed to be hard-wired to 512 bytes per sector in calculations (see Advanced Format), that is, there can be more than four partition entries in a single sector, and (with possible future much larger partition table entries) it is possible to have a sector hold only a fraction of a partition entry. Except for the first two sectors (LBA 0 and LBA 1), the GPT specification just describes the size and organization of a data structure, not in how many sectors it is stored on disk.

The 64-bit partition table attributes are shared between 48-bit common attributes for all partition types, and 16-bit type-specific attributes.

Partition attributes

Bit

Content

0

System partition (disk partitioning utilities must preserve the partition as is)

1

EFI firmware should ignore the content of the partition and not try to read from it

Hybrid MBRs are non-standard and can be interpreted in different ways by different OSes.[10] Unless otherwise noted, OSes provide precedence to the GPT data when a hybrid MBR configuration is encountered.

The term No native support on this arch and version. should be understood this way:

Not supported as data disk,[11] only known legacy partitions found in protective MBR are accessible via the OS. Detachable disks: only support for MBR partitioning; No access with end user applications. GPT contained raw data is accessible with third-party administrator tools for low level disk access. True file system level support in read or read-write form might be subject of software from a third-party vendor.

^In a multi-disk setup, non-UEFI bootloader (boot drive) requires MBR-based partitioning, while a system drive can use GUID partitioning.

^The GUIDs in this table are written assuming a little-endianbyte order. For example, the GUID for an EFI System partition is written as C12A7328-F81F-11D2-BA4B-00A0C93EC93B here, which corresponds to the 16 byte sequence 28h 73h 2Ah C1h 1Fh F8h D2h 11h BAh 4Bh 00h A0h C9h 3Eh C9h 3Bh – only the first three blocks are byte-swapped.

^The formation of this GUID does not follow the GUID definition; it is formed by using the ASCII codes for the string "Hah!IdontNeedEFI". Such formation of "GUID" value breaks down the guaranteed uniqueness of GUID.

^ abSome computer manufacturers have their own GUIDs for partitions that are analogous to the EFI System Partition, but that hold boot loaders to launch manufacturer-specific recovery tools.[25]

^ abPreviously, Linux used the same GUID for the data partitions as Windows (Basic data partition: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7). Linux never had a separate unique partition type GUID defined for its data partitions. This created problems when dual-booting Linux and Windows in UEFI-GPT setup. The new GUID (Linux filesystem data: 0FC63DAF-8483-4772-8E79-3D69D8477DE4) was defined jointly by GPT fdisk and GNU Parted developers. It is identified as type code 0x8300 in GPT fdisk. (See definitions in gdisk's parttypes.cc)

^ abThe GUID for /usr on Solaris is used as a generic GUID for ZFS by Mac OS X.

^ abNetBSD and MidnightBSD had used the FreeBSD GUIDs before their unique GUIDs were created.

^ abcdefThe Ceph filesystem uses GUIDs to mark the state of preparation a disk is in.[33]

^Nikkel, Bruce J. (September 2009). "Forensic analysis of GPT disks and GUID partition tables". Digital Investigation6 (1-2): 39–47. doi:10.1016/j.diin.2009.07.001. The current popular BIOS and MBR partitioning scheme was originally developed in the early 1980s for the IBM Personal Computer using IBM PC-DOS or MS-DOS. The Basic Input/Output System (BIOS) provides an interface to the hardware and initiates the boot process (IBM, 1983). The MBR, located in sector zero, contains the initial boot code and a four entry partition table (Microsoft, 1983). Intended to solve booting and partitioning limitations with newer hardware, a replacement for both the BIOS and the MBR partition table was developed by Intel in the late 1990s (Intel, 2000). This is now called the Unified EFI (UEFI, 2008 UEFI Forum. Unified extensible firmware interface specification version 2.2 2008.UEFI, 2008) specification, and managed by the UEFI Forum (UEFI, 2009). A subset of this specification includes GPT, intended to replace the DOS/MBR partition tables.